Platinum based catalysts are used for numerous hydrocarbon conversion processes. In many applications promoters and modifiers are also used. One such hydrocarbon conversion process is the dehydrogenation of hydrocarbons, particularly alkanes such as isobutane, which are converted to isobutylene. For example, U.S. Pat. No. 3,878,131 (and related U.S. Pat. Nos. 3,632,503 and 3,755,481) discloses a catalyst comprising a platinum metal, a tin oxide component and a germanium oxide component. All components are uniformly dispersed throughout the alumina support. U.S. Pat. No. 3,761,531 (and related U.S. Pat. No. 3,682,838) discloses a catalytic composite comprising a platinum group component, a Group IVA metallic component, e.g., germanium, a Group VA metallic component, e.g., arsenic, antimony and an alkali or alkaline earth component all dispersed on an alumina carrier material. Again all the components are evenly distributed on the carrier.
U.S. Pat. Nos. 3,558,477, 3,562,147, 3,584,060 and 3,649,566 all disclose catalytic composites comprising a platinum group component and a rhenium component on a refractory oxide support. However, as before, these references disclose that the best results are achieved when the platinum group component and rhenium component are uniformly distributed throughout the catalyst.
It is also known that for certain processes selectivity towards desirable products is inhibited by excessive residence time of the feed or the products at the active sites of the catalyst. Thus, U.S. Pat. No. 4,716,143 describes a catalyst in which the platinum group metal is deposited in an outer layer (about 400 .mu.m) of the support. No preference is given to how the modifier metal should be distributed throughout the support. Similarly U.S. Pat. No. 4,786,625 discloses a catalyst in which the platinum is deposited on the surface of the support whereas the modifier metal is evenly distributed throughout the support.
U.S. Pat. No. 3,897,368 describes a method for the production of a noble metal catalyst where the noble metal is platinum and the platinum is deposited selectively upon the external surface of the catalyst. However, this disclosure describes the advantages of impregnating only platinum on the exterior layer and utilizes a specific type of surfactant to achieve the surface impregnation of the noble metal.
The art also discloses several references where a catalyst contains an inner core and an outer layer or shell. For example, U.S. Pat. No. 3,145,183 discloses spheres having an impervious center and a porous shell. Although it is disclosed that the impervious center can be small, the overall diameter is 1/8" or larger. It is stated that for smaller diameter spheres (less than 1/8"), uniformity is hard to control. U.S. Pat. No. 5,516,740 discloses a thin outer shell of catalytic material bonded to an inner core of catalytically inert material. The outer core can have catalytic metals such as platinum dispersed on it. The '740 patent further discloses that this catalyst is used in an isomerization process. Finally, the outer layer material contains the catalytic metal prior to it being coated onto the inner core.
U.S. Pat. No. 4,077,912 and 4,255,253 disclose a catalyst having a base support having deposited thereon a layer of a catalytic metal oxide or a combination of a catalytic metal oxide and an oxide support. WO98/14274 discloses a catalyst which comprises a catalytically inert core material on which is deposited and bonded a thin shell of material containing active sites.
Applicants have developed a layered catalyst composition which differs from the prior art in several ways. The composition comprises an inner core such as alpha alumina and an outer layer such as gamma alumina. The outer layer has uniformly distributed thereon at least one platinum group metal such as platinum and a modifier metal such as tin. The platinum group metal to modifier metal atomic ratio varies from about 0.1 to about 5. The outer layer has a thickness of about 40 to about 400 microns. A modifier metal, e.g., lithium, is also present on the catalyst composition and may be present either entirely in the layer or distributed throughout the catalyst composition. Finally, the composition is prepared using an organic bonding agent such as polyvinyl alcohol which increases the bond between the layer and the inner core thereby reducing loss of the layer by attrition.